Method and apparatus for curing a composite article

ABSTRACT

Disclosed is a method of curing a composite article and associated curing apparatus. A heat source is provided for heating the composite article. A temperature related property is detected proximal to the heat source and the heat output is regulated to a predetermined temperature vs. time profile. Heat output vs time data is acquired and functionalised and the curing completion time is determined based on the functionalised heat output vs time data.The method provides for heating a composite article so as to follow a predetermined temperature vs. time profile (i.e. a cure profile) and avoid excessively high temperatures. The required heat output vs time has also been found to be broadly reproducible, and the curing completion time can be more readily determined from functionalised heat output vs time data, for example by identifying reproducible characteristics of the functionalised data.

FIELD OF THE INVENTION

The invention relates to methods and heated tools for curing articlesformed from composite materials such as carbon fibre, in particular foraerospace applications.

BACKGROUND TO THE INVENTION

Composite materials are widely used in a number of industries, such asaerospace, automotive, civil engineering and sports goods, due to theirhigh strength-to-weight ratio.

Carbon fibre composite material (also known as carbon fibre reinforcedpolymer, or simply “carbon fibre”), is an example of a compositematerial formed from a fabric or fibrous component reinforced by apolymeric or resin component.

Carbon fibre composite material is formed from layered carbon fibrefabric impregnated with and reinforcing a polymer matrix. The fabric isformed by carbonizing a synthetic polymer fabric material and may beprovided in woven, non-woven or may consist of unidirectional fibres.The carbon fibre fabric provides the majority of the compositematerial's strength, whilst the resin provides additional strength andrigidity and also protects and preserves the mechanical properties ofthe carbon fibres.

The polymer matrix is typically a thermoset polymer, formed from aresin; commonly an epoxy resin. Whilst the formation of a thermosetpolymer from a resin precursor is normally an exothermic reaction, epoxyresin systems used for large scale industrial applications normallyrequire the input of thermal energy to initiate curing, i.e.cross-linking between polymeric chains within the resin, to thereby formthree-dimensional polymer matrices.

Conventionally, following lay-up of the fabric and resin impregnation,carbon fibre articles are cured in an autoclave, under high temperature(ca. 100-300° C.) and pressure (up to 7 Bar).

Whilst this method of curing is effective, it is associated with highcapital expenditure, and high operating costs.

Particularly for the aerospace industry, there has been increasingadoption of so called “out of autoclave” (OOA) curing methods, in whicha carbon fibre article is held at a reduced pressure (in a vacuum bag)and cured on a heated tool, or between heated tools.

The initial and operating costs for OOA manufacture are significantlyreduced, but problems can be encountered in controlling the curingreaction; in terms of ensuring complete curing throughout large itemsand/or preventing localised overheating, which can adversely affect thefinal structural performance of a composite article.

The relevant conditions across the face of a curing tool will bedifferent for each composite article and so, conventionally, OOA curingconditions are determined experimentally, based on differential scanningcalorimetry or destructive testing. However, during manufacture, curingcannot be effectively monitored. Due to inherent variations betweenparts, in ambient conditions, batches of carbon fibre and/or resin andthe like, curing conditions must therefore be set conservatively (slowramp rates, long dwell times) to ensure that overheating or incompletecuring are avoided. This adds to manufacturing time and energyconsumption.

In U.S. Pat. No. 8,473,093, Gershenfeld et al. describe how a network of“nodes” might be embedded in an OOA curing tool and subject to closedloop control by a processor, where each node comprises a thermistor orthermocouple, and a heating element. The processor controls the heatinginput based on detected temperature, in order to follow a predeterminedcuring temperature profile.

It has been proposed to embed means for detecting temperature within thecomposite articles themselves, so as to more accurately monitortemperature deep within a composite structure. However, this adds to thecost and complexity of manufacture, and for many applications it is notdesirable for the thermocouple/thermistors to be present in the finalcomposite article.

In their proof of concept work, Gershenfeld et al. demonstrate that inprinciple, a signature of an exothermic curing reaction taking placewithin a cup of an epoxy resin can be followed based on the excursion ofrequired heating input from that expected, based on comparative tests ofan already cured sample. However, this signature was seen in testsconducted using a single node on a sample of bulk resin, and theprinciple was not tested on a composite material in a curing tool, wherethe signal would be far lower.

Accordingly, there is a need for improved monitoring and control ofcuring during OOA composites manufacture.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a methodof curing a composite article, comprising;

-   -   providing a heat source for heating the composite article;    -   detecting a temperature-related property proximal to the heat        source;    -   regulating the heat output of the heat source to the composite        article, based at least in part on the detected        temperature-related property, to a predetermined temperature vs.        time profile;    -   acquiring heat output vs time data; and    -   functionalising heat output vs time data; and    -   determining a curing completion time based on the functionalised        heat output vs time data.

The method provides for heating a composite article so as to follow apredetermined temperature vs. time profile (i.e. a cure profile) andavoid excessively high temperatures.

Deviations observed between the expected required heat output and theactual required heat output to follow a cure profile can be attributedto enthalpy of the curing reaction. The required heat output vs time hasalso been found to be broadly reproducible, but subject to experimentalvariations, caused by variations in the amount of resin and the densityof the composite article, ambient conditions, the duty cycle of aheating tool, and the like.

It has been found that the curing completion time can be more readilydetermined from functionalised heat output vs time data (hereinafterreferred to as the “functionalised data” or “F(t)”), such as the firstderivative of heat output vs time data, than from the raw heat output vstime data. The functionalised data can therefore assist in moreaccurately or reliably determining the curing completion time.

The heat output may be a value of an electrical current or powersupplied to the heat source.

The method may comprise identifying a reproducible characteristic of thefunctionalised data, and determining the curing completion time based onthe reproducible characteristic.

The functionalising may comprise applying one or more (mathematical)functions to the heat output vs time data.

The functionalised data may be a first derivative of the heat output vstime data (the “first derivative”) or a second derivative of the heatoutput vs time data or indeed a higher order derivative. Thereproducible characteristic of the functionalised data may be the saidderivative approaching zero. Variations in the offset, i.e. the absoluteheat output required to maintain a temperature, thereby need not beaccounted for.

For example, the curing completion time may be determined by detectingwhen the first derivative remains within a threshold range of zero for apredetermined period. The threshold range may be within ±0.1, within±0.05, within ±0.01, within ±0.005 or within ±0.001. It will beunderstood that a smaller threshold range will typically correspond to ahigher overall degree of curing of the composite material, wherein theacceptable degree of curing will be dependent on each particularapplication.

The functionalised data may be a square or higher order power, or anysuitable mathematical processing suitable to enhance or exaggerate acharacteristic of the heat output vs time data.

The functionalised data may be a result of a change detection methodapplied to the heat output vs. time data, such as a Bayesian changepoint detection algorithm, k-mean clustering using kurtosis (where eachcell would correspond to a particular stage in the cure cycle), subspacemodelling, probabilistic methods, machine learning techniques, or othersuitable change detection methods as known in the art.

The functionalised data may comprise a combination of these approachessuch as the result of a change point analysis applied to a said first orsecond derivative.

The curing completion time may be determined by detecting a reproduciblecharacteristic of the functionalised data corresponding to a localmaximum or minimum, steepest gradient, or inflection point in the poweroutput vs. time. For example, the curing completion time may bedetermined by detecting a corresponding local maximum or minimum in aderivative, such as the first derivative, of heat output vs time data.

It has been observed that such a reproducible characteristic, such as alocal maximum or minimum, occurs at a predictable time before curing iscomplete and so its detection can be used to predict the curingcompletion time. Accordingly, the method may comprise detecting areproducible characteristic of the functionalised data (e.g. a localmaximum or minimum in the first derivative) and predicting the curingcompletion time.

The method may comprise smoothing the heat output vs time data, eitherbefore or more after the functionalisation. Smoothing of the data mayassist dealing with noise in the data. Detecting the reproduciblecharacteristic may be performed on the smoothed data.

The method may for example comprise calculating a rolling average,example each point of the smoothed data being an average of a pre-setnumber of preceding data points. For example where data is collectedevery second or every ten seconds, each datum of the smoothed data (bethat the heat output vs time data, or the functionalised data) may be anaverage of the preceding 5, or 10, or more data points.

The method may comprise acquiring data for an evaluation period and thenaveraging the acquired data. This may be repeated for one or moresubsequent evaluation periods, such that each datum of the smoothed datais a smoothed data block that extends for an evaluation period in thetime domain, and has a value of an average of the data acquired over thepreceding evaluation period.

In some embodiments, the method comprises acquiring the first derivative(or higher order derivative, as the case may be) of the heat output vstime for an evaluation period, averaging the first derivative over theevaluation period to obtain a smoothed data block, and repeating until areproducible characteristic is observed in the smoothed data.

The reproducible characteristic may for example be a smoothed data blockhaving a first derivative value within a threshold range of zero.

In some embodiments, the reproducible characteristic may be a sequenceof smoothed data blocks having values indicative of a local maximum orminimum.

A reproducible characteristic may be a local minimum of smoothed datablocks, which may for example be characterised by a data block having afirst derivative value lower than the preceding and the following datablocks. Similarly, a local maximum may be characterised by a data blockhaving a first derivative value higher than the preceding and followingdata blocks. A local minimum or maximum may be additionally oralternatively characterised by a data block having a first derivativevalue higher or lower than a threshold value.

Further criteria may be optionally applied when identifying areproducible characteristic, for example whether the preceding andfollowing data blocks are respectively higher/lower than a given datablock by more than a predetermined amount; such as by more than 0.01, ormore than 0.05. The method may comprise determining trends in a greaternumber of data blocks, for example where the derivative value decreasesfor two or more successive data blocks and subsequently increases fortwo or more successive data blocks (or vice versa).

The evaluation period may for example around 1 minute, 5 minutes or 10minutes. The evaluation period may in some embodiments be around 9minutes.

Advantageously, the evaluation period is only a relatively smallproportion of the length of the standard cure cycle (such as a curecycle recommended by a resin manufacturer to ensure complete curing).For example, the evaluation may be less than around 15% or 10%, oraround 7.5% (or less) of a normal curing cycle, and thus provides forsignificant time savings.

The method may comprise providing more than one heat source and morethan one temperature sensor (which together may be referred to as a“node”).

In embodiments having multiple nodes, detecting or predicting the curingcompletion time may comprise identifying a reproducible characteristicin the data from more than one node.

The nodes may be grouped. The curing completion time for the group ofnodes may in some embodiments be considered to have been identified onlywhen a said reproducible characteristic has been identified in data fromall of the nodes in a group.

In some embodiments, nodes in corresponding locations on opposed mouldsmay be paired, such that the curing completion time for the pair ofnodes is considered to have been identified only when a saidreproducible characteristic has been identified in data from both nodes.

The curing completion time may be predicted using a combination of theforegoing. The method may comprise refining a predicted curingcompletion time. For example, the curing completion time may bepredicted based on detecting a reproducible characteristic, such as alocal maximum or minimum in a derivative of heat output vs time data,and may be subsequently refined (or confirmed) based on the detection ofa further reproducible characteristic, such as a derivative within athreshold range of zero for a predetermined period of time.

The curing completion time may be when the composite is known (e.g. fromdestructive testing, spectroscopic analysis, differential scanningcalorimetry or any other suitable testing method) to be at least 90%, or95% or at least 98% or 99% cured.

The method may comprise cooling the composite article based on thedetermined curing completion time.

The cooling may commence after the curing completion time.

The method may comprise commencing the cooling a predetermined timeperiod after the curing completion time. For example, a predeterminedtime may be allowed to elapse following the curing completion time,before the composite article is cooled.

This may be required to ensure that curing is complete throughout thecomposite article, particular for large composite articles and/or wherethe heat source and temperature detection is external to the compositearticle. However, in some embodiments it has been found that the heatcapacity of a composite article (even for relatively thick compositearticles, comprising 20 or more plies of carbon fibre) is such thatsufficient heat is stored to cure all parts of the article, even whencooling is commenced at the determined curing completion time.

In some embodiments, where the curing completion time has beenpredicted, the method may comprise commencing cooling before the curingcompletion time.

The cooling may be in accordance with a predetermined cure profile.

The composite article may be cooled by reducing the output of, orturning off, the heat source, and allowing the composite article tocool.

The heat output of the heat source may be regulated by closed loopcontrol, such as PID control. The control may be effected by aprocessing resource, such as a computer processor running software offirmware. A processing resource may be local to the heat source, or theheat source may be controlled remotely (e.g. over a wireless or wirednetwork). Indeed, the method may comprise use of a distributedprocessing resource.

The method may comprise detecting a temperature related property of atemperature sensor, e.g. an electrical signal such as a voltage,current, or resistance. Any suitable temperatures sensor may be used,such as a thermocouple or thermistor.

The method may comprise detecting or calculating a temperature from thetemperature-related property.

The skilled addressee will understand that regardless of whether atemperature value is calculated, a temperature-related property can becorrelated with temperature, so as to enable the predeterminedtemperature vs time profile (cure profile) to be followed.

The method may comprise detecting a temperature-related property at orwithin a curing tool. For example a temperature sensor may be attachedto or embedded in a curing tool

The method may comprise detecting a temperature-related property on asurface of or within a composite article. For example, a temperaturesensor may be attached to a surface of or embedded within a compositearticle (e.g. during lay-up).

In some embodiments, the method comprises detecting atemperature-related property using the heat source. For example, aheating element can in some embodiments also function as a temperaturesensor.

The method may comprise providing more than one heat source, anyplurality of heat sources. The method may comprise the steps ofdetecting, regulating, monitoring and determining as described herein,in relation to each said heat source.

The method may be used with any suitable type of heat source, and maycomprise inductively heating, resistively heating, radiatively heatingand/or microwave heating the composite article.

A heat source may be provided embedded within or attached to a curingtool. A heat source may be embedded in or attached to the compositearticle. In some embodiments, the composite article may be electricallyconductive and be directly resistively or inductively heated; in effectthe composite article acting as a heat source.

For example, a composite article may comprise one or more conductivelayers, such as reinforcing metallic members or mesh layers. Certaincomposites, including carbon fibre, are based upon a conductivereinforcing fibre.

A plurality of heat sources may be independently subject to closed loopcontrol, as disclosed herein.

The method may comprise providing the composite article, for examplecomprising laying up fabric layers, impregnating with resin, debulking,preforming and other such steps for composite manufacture, known in theart. The method may comprise placing an uncured composite article on acuring tool and/or removing it therefrom after curing.

By a composite article we include an article formed from or comprising areinforcing fibrous or particulate material and a curable matrixmaterial. The reinforcing material can include a woven or non-wovenfabric material, or alternatively reinforcing fibres may be dispersedwithin a matrix material. Any suitable reinforcing material, orcombination of reinforcing materials, may be used, including carbonfibre, glass fibre, aramid, basalt, porous materials such as porousceramics or plastics materials. Moreover the method may be applied toany suitable curable matrix material, including epoxy resins, polyesterresins, vinylester resins, alkyd resins; and is equally applicable toexothermic or endothermic curing reactions. Composite articles may alsoinclude other components embedded or encapsulated within or attached tothe reinforcing material/curable matrix material, such as honeycombstructures or metallic couplings or reinforcements.

The method is of particular utility in the curing of carbon fibrecomposite articles, and can be used to cure articles composed ofpre-impregnated or semi-impregnated carbon fibre fabric, or where resinis introduced following lay-up of the reinforcing fabric.

By a curing tool we refer to the apparatus used to cure a compositearticle, by heating. A curing tool typically comprises a mould defininga surface or surfaces of the composite article, which may be providedwith a heat source(s) as described herein. A curing tool may comprisemore than one mould, commonly two opposed moulds; one or both of whichmay be heated.

Where we refer to detection of a temperature-related property proximalto the provision of a heat source, or a temperature sensor beingproximal to a heat source, we mean spatially proximal, so that thetemperature or temperature-related property is representative of orcorrelated to the effect of the heat source on the curing of a compositearticle in the vicinity of the heat source. Proximal includes in thesame location, within the same unit or node and can also include one orother of the heat source or detection of a temperature-related property(e.g. by a temperature sensor) being in a curing tool and the otherbeing in or of the composite article.

According to a second aspect of the invention there is provided curingapparatus for curing a composite article, the curing apparatuscomprising;

-   -   a curing tool, having at least one mould;    -   a heat source for heating a said composite article;    -   a temperature sensor for detecting a temperature-related        property proximal to the heat source; and    -   a control arrangement configured to;        -   regulate the heat output of the heat source to the composite            article, based at least in part on the detected            temperature-related property, to a predetermined temperature            vs. time profile;        -   acquire heat output vs time data;        -   functionalise the heat output vs time data; and        -   determine a curing completion time based on the            functionalised heat output vs time data.

The control arrangement may comprise one or more controllers, aprocessing resource and data storage.

The data storage may be used to store and retrieve acquired heat outputvs time data, and/or functionalised data as described below.Experimental parameters such as cure profiles, look-up tables for knowncomposites and the like may be stored on the data storage and retrievedtherefrom.

The processing resource may be configured to functionalise the heatoutput vs time data. The invention is not limited to any particularsoftware or hardware architecture, but may for example comprise one ormore processing modules for functionalising, and otherwise processingthe heat output vs time data.

A functionalising module may be configured to functionalise the heatoutput vs time data, for example to apply one or more mathematicalfunctions to heat output vs time data, such as a first or higher orderderivative, a square or higher power or any suitable mathematicalprocessing suitable to enhance or exaggerate a characteristic of theheat output vs time data, as disclosed herein.

The processing resource may comprise a smoothing module. The smoothingmodule may smooth the heat output vs time data, or the functionaliseddata.

The smoothing module may be configured to calculate a rolling average ofthe heat output vs time data, or the functionalised data. The smoothingmodule may be configured to average acquired heat output vs time data,or functionalised data, over an evaluation period, or for one or moresubsequent evaluation periods, wherein each datum of the smoothed datais a smoothed data block that extends for an evaluation period in thetime domain, and has a value of an average of the data acquired over thepreceding evaluation period.

The processing resource may comprise a characterising module, configuredto determine the curing completion time, based on the functionaliseddata (which optionally has been smoothed). Determining the curingcompletion time may comprise identifying a reproducible characteristicof the functionalised data, as disclosed in relation to the firstaspect.

The various modules may send or receive data from one another or thedata storage, as required.

It will be understood that the processing resource may comprise acomputer processor, or its functions may be performed by more than onecomputer processor, which may be local to a heat source, central to thecuring apparatus, or provided remotely, across a network. Indeed theprocessing response may comprise more than one processor, such as anetwork of processors. The various modules disclosed herein may each beprovided by a processor or more than one processor. The modulesdisclosed herein may be provided in the form of software executed on aprocessor or processors. Similarly, data storage may be local to a heatsource, central to the curing apparatus, or data may be stored remotely,e.g. across a network.

The apparatus may comprise more than one, or a plurality of heat sourcesand temperature sensors. The apparatus may comprise a number or array ofnodes, each node comprising a heat source and temperature sensor.

The curing tool may comprise said the or each heat source andtemperature sensor. The curing tool may comprise said nodes.

The respective heat sources and temperature sensors may each be providedwith a controller. Each controller may be operable independently as acontrol arrangement, operable to regulate, monitor and determine asdisclosed herein. Alternatively, a control arrangement may be associatedwith more than one heat source or temperature sensor. For example, acontrol arrangement may be associated with a heat source and one or moreneighbouring heat sources. A given heat source and/or temperaturesensor, may be associated with more than one control arrangement. Forexample, a control arrangement may control a heat source, takingreadings from a neighbouring heat source and/or temperature sensor intoaccount.

Each node may comprise a control arrangement, or a processing resourceor data storage;

or form a part thereof. Each node may comprise a controller, such as aclosed loop controller, e.g. a PID controller. Alternatively, suchcontrol may be exerted by a control arrangement common to more than onenode, such as a central control arrangement.

The nodes may be networked together, for example in order to share acommon data storage, processing resource, power supply and the like.

The mould may be composed of any suitable material. The mould may forexample be metallic, of have a metallic mould surface. Alternatively,the mould may comprise a composite mould surface.

The or each heat source and/or temperature sensor may be attached to,recessed in or embedded in the mould. For example, a mould may have aheat source embedded close to the mould surface. In some embodiments, itmay be convenient for a heat source or a node to be attached to a mould.

A curing tool may alternatively be configured to communicate with a heatsource and temperature sensors, such as those embedded in or forming apart of the composite article itself.

The apparatus may comprise any suitable type or types of heat source,and may comprise a heat source for inductively heating, resistivelyheating, radiatively heating and/or microwave heating the compositearticle.

Conveniently, the heat source is a heating element. In some embodiments,the heating element can in some embodiments also function as atemperature sensor.

The apparatus may comprise more than one mould. For example theapparatus may comprise opposed moulds, such as an upper and a lowermould, each of which may be provided with one or more heat sources,temperature sensors or nodes, as the case may be.

Alternatively, the apparatus may comprise a single mould and aninsulating cover.

The apparatus may be adapted to cure a composite article within anautoclave, but both the apparatus and the method is of particularutility in out of autoclave composite curing application. Accordingly,the curing tool may be an out of autoclave (OOA) curing tool, and theapparatus may comprise such additional equipment required for OOAapplications, such as a vacuum bag, a vacuum pump and so forth.

The apparatus may comprise or be connectable to a user interface, inorder to change parameters such as processing parameters, cure profileand the like.

The invention extends in a further aspect to a node for a curingapparatus, comprising a said heat source, temperature sensor andcontroller. Optionally the node comprises a processing resource, datastorage and/or a closed loop controller. The node may be configured tointerface with a control arrangement, such as a computer processor,operable to perform the method of the first aspect in conjunction withone or more nodes. The invention thus extends to a said controlresource, and to computer executable programme code (stored in anysuitable format) which may be run on a computing device to cause it tofunction as a control arrangement as disclosed herein.

The control arrangement may be configured to execute any or all of thefeatures of the method of the first aspect. Indeed preferred or optionalfeatures of the each aspect of the invention correspond to preferred andoptional features of the each other aspect of the invention.

DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to thefollowing figures in which:

FIG. 1 shows (a) a schematic cross sectional view and (b)/(c) images ofa test curing tool.

FIG. 2 is a schematic of the architecture of a control arrangement thatcommunicates with a heat source and temperature sensor of a curing tool.

FIG. 3 is a schematic cross section of the tool layup of a curing tool.

FIG. 4 is an example cure profile.

FIG. 5 shows power vs time data for a node of a test curing tool duringa cure cycle and a re-cure cycle. A difference plot between the twocycles is also shown.

FIG. 6 shows power vs time data for another node of the test curing toolduring a cure cycle, together with smoothed first derivative data.

FIG. 7 shows a schematic cross sectional view of the tool layup of atwo-sided curing tool.

FIG. 8 shows (a) power vs time data and (b)-(d) smoothed firstderivative of the power vs time data, for a selected node of the twosided tool of FIG. 7. Data for the corresponding node of the upper andlower tools is presented.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

A one-sided test tool was used to obtain data on curing of testcomposite articles, to demonstrate the principles of the invention.

A test curing tool is shown in FIG. 1(a)-(c). The tool 1 is formed of aglass fibre plate 3 (a mould), to act as a mould for curing a testcomposite article, with heat sources in the form of eleven embeddedsilicon heater pads 5 (an example of a heat source) and elevencorresponding thin film PT100 resistive temperature sensors 7 (alsoreferred to as resistive temperature detectors, RTDs).

As shown in the schematic cross sectional view of FIG. 1(a), the heaterpads 5 and the RTDs 7 are located close to the mould surface 6 of theplate 3. The RTDs 7 are centred above each heater pad 5 to facilitateregulation of the heat output of the respective pad. The selected RTDsprovide an accurate and repeatable resistance signal that can be relatedto temperature (i.e. resistance of the RTD 7 being a temperature-relatedproperty) within the target temperature range of 0-200° C. associatedwith curing of epoxy resin-based carbon fibre composites.

The position of the heat sources is marked on the mould surface 6 (seeFIG. 1(b)). FIG. 1(c) shows the tool 1 after a protective PTFE layer hasbeen applied to the mould surface to assist in release of the compositearticles during testing.

The plate consists of 16 plies of woven glass fibre infused with anepoxy resin. Glass fibre was selected to prevent electrical shorting ofembedded components during manufacture. The heater pads 5 and RTDs 7 arelocated 1 ply below the heating surface, so that thermal changes withina composite article in the tool 1 can be detected quickly andaccurately.

In the embodiment shown, an individual heater pad surface measured25.4×127 mm and 0.2 mm thick, creating a 127×289.4 mm total surface areawith 1 mm gap between adjacent pads. The pads were rated at 1.6 W/cm²and used a 240V AC supply.

Each of the heater pads 5 and corresponding RTDs 7 is connected to arespective microcontroller located on the underside of the tool 1 (notvisible in FIG. 1). In effect, therefore, the tool 1 comprises a seriesof nodes 2 formed from a heat source, temperatures sensor and, in thisembodiment, a microcontroller.

The architecture of each node 2 is shown in FIG. 2. The controlarrangement includes a microcontroller and the relevant software runningon a computer processor. The microcontroller 11 is configured toimplement Proportional-Integral-Derivative (PID) control to regulate theoutput of the corresponding heat source 5. The microcontroller 11schedules the supply of AC power to the heater pad 5 based on feedbackfrom the RTD 7. This is a form of closed loop control, but other meansof closed loop control may also be used.

Each node 2 communicates with a processing resource 13, which, in theembodiment shown is in the form of a PC connected via a Universal SerialBus (USB) interface. The PC is provided with a Graphical User Interface(GUI), by which a user is able to define a cure profile (i.e. atemperature vs time profile) that is then communicated to and locallystored at each individual microcontroller 11.

In use, and as described in further detail below, each microcontrolleris operable to regulate the heat output of its heat source, to followthis predetermined cure profile. The cure profile creates a series oftemperature set points over time for the microcontroller 11 to drive theheater pads 5 to. The RTD temperature, power consumption and PID valuesare logged to .csv files, which are stored on the PC.

The microcontrollers used are 8-bit Atmel ATMega 328 with fiveAnalogue-to-Digital Converters 10 (ADC) and five pulse width modulation(PWM) outputs. Although in the embodiment shown each microcontroller 11is associated with a single temperature sensor 7 and heat source 5,multiple inputs and outputs providing the ability to implement multiplezone controllers with the same microcontroller, in alternativeembodiments.

The embedded ADCs are single-ended successive approximations and theirchannels are multiplexed. They provide 10-bit resolution with anabsolute accuracy of 2 Least Significant Bits (LSB).

The power supplied to each of the heater pads 5 is regulated using azero crossing detector circuit. When the AC sine wave crosses the 0 Vaxis, a 1 mV pulse is sent from a zero crossing optocoupler or scheduler(ZCS) 12 to the microcontroller 11. The ZCS 12 regulates the AC supplyto the heater pad 5 based on a duty cycle determined by themicrocontroller 11. The number of pulses can be tracked and the powersupply can be turned on and off in for different periods of time, in asimilar method to pulse width modulation.

The ADC 10 of the microcontroller 11 is single-ended and utilizes asignal conditioner 14 that consisted of three parts:

-   -   Constant current source to bias the RTD with a constant current        of 1 mA to prevent self-heating which could disrupt the        temperature-resistance relationship.    -   Bridge to compensate for wire heating effects connected to the        RTD.    -   Amplification stage to amplify the bridge output for better        voltage per ADC value (0-1023) on the microcontroller.

The microcontroller's PID compares the current temperature of the RTD tothe set point at a given time. Depending on the error margin, the PIDsends an appropriate zero-scheduling signal to the heater pad drivercircuit. The PID was tuned using the Zeigler-Nichols method using noovershoot parameters. K_(i) and K_(d) were set to 0 and K_(d) wasincreased until the system oscillates around the set point with constantamplitude of K_(u) and an oscillation period of T_(u).

K_(u) is the proportional gain coefficient used during the tuningprocess, and is the value of the proportional coefficient (K_(p)) thatis found to produce a constant oscillation around a given set point whenthe integral coefficient (K_(i)) and the derivative coefficient (K_(d))are set to zero.

T_(u) is the oscillation period corresponding to a proportional gaincoefficient of K_(u)

From the values of K_(u) and T_(u), the true K_(p), K_(i) and K_(d)values were derived using set ratios as described in the Zeigler-Nicholsmethod. This was used to establish baseline PID coefficients (K_(p),K_(i), K_(d)) which could then be adjusted by the global controllerbased on experimental analysis.

Gain scheduling was used to adjust PID gains depending on the currentsetpoint. For example when the temperature of the plate was similar tothe ambient temperature, less aggressive PID values were required totrack the set point. In use of the tool 1, as temperature increases, thetemperature difference between the plate and the surrounding atmosphereincreases, requiring more aggressive PID values to be able to track theset point.

The PID controller 16 was able to maintain the set point temperaturewithin ±0.5° C. This degree of accuracy was observed for all 11 zones ornodes within the tool 1.

Experimental

The following methodology was used for all experiments.

Test Composite Articles

Test composite articles were prepared as follows: Aerospace grade wovenpre-preg carbon fibre was used to create 18 ply thick 127×289.4 mmpanels. 18 plies were used to create a part with a representativethickness of an aerospace component.

The selected pre-preg carbon fibre required a cure profile comprising atemperature ramp of 3° C./min from ambient to 180° C., followed by adwell period at this temperature.

Following lay-up, the panels were stored in a laboratory freezer withinvacuum bags and were removed 24 hours before curing to allow completedefrosting.

Panel Positioning and Instrumentation

The apparatus setup for curing a test panel 15 (collectively known asthe “tool layup”) is shown in FIG. 3.

A panel 15 is centered on the curing tool 1 and encased in two layers ofPTFE “peel paper” 17, to prevent bonding to the mould surface 6 of thetool 1.

For the purposes of the test experiments, two RTDs was taped using PTFEcoated release film on the top surface of the panels, to providetemperature measurements for the top surface of the plate. These are notshown in the figures.

Cure Layup

Curing of the panel requires application of a 99% vacuum, to minimisevoids and to compress the carbon fibre layup.

A layer of woven polymer breather 18 was placed over the test panel 15to facilitate air flow, and a flexible vacuum “bag” 20 placed over thetest panel and breather so as to contact the mould surface 6 around theperiphery of the test panel 15. A tube 21 connects between the vacuumbag 17 and a vacuum pump 19.

Curing

After vacuum was applied, each test panel 15 was cured using the cureprofile shown in FIG. 4; i.e. an initial equilibration period at 30° C.,a 3° C./min temperature ramp to 180° C., a dwell time of 120 minutes anda temperature ramp of −3° C./min to 30° C., after which the apparatuswas switched off and allowed to cool for 4 hours.

Small core samples were removed from each test panel following curing,which were subject DSC analysis to verify that ≥99% curing had takenplace, and the cure profile was then repeated for each test panel toobtain comparative data.

The experimental protocol was conducted on six test panels to assessrepeatability.

MATLAB (a trademark of MathWorks, Massachusetts, USA) was used for dataanalysis and visualisation. The data from each RTD was separated anddisplayed on 11 different graphs, representing the different heaternodes. For the purposes of following, data for individual nodes will bepresented.

Results

An example of the power consumption over time of the heat source of asingle node during the cure and then re-cure is shown in FIG. 5. Thepower signals were averaged over 1000 data points using a movingSavitzky-Golay filter (i.e. smoothed data is presented).

For the cure cycle (solid line 23), the power output of the heat sourceis presented as a percentage of maximum output. Data for heater number 4is presented in the figure, but comparable data were obtained for otherthe heaters. Power output initially settles during the equilibrationperiod at 30° C.

During the temperature ramp at 3° C./min, the power steadily increasesuntil peaking at approximately 22% at t=55 mins.

As expected, the power output to maintain the temperature during thedwell period at 180° C. was lower than that required during thetemperature ramp.

The power output passes through a local minimum 25 at t≈78 mins and thenrises towards a steady rate of around 17.5% at t≈120 mins, where itremains for the rest of the dwell period. The power consumption thenduring temperature ramp down at −3° C./min.

During the re-curing cycle (dotted line 27), the power output isinitially higher the cure profile power consumption and continues todiverge. The power output peaks at 25% at t≈22 min, before fallingtowards a same steady state of around 17.5% at t≈120 min. Thereafter,power output is as during the cure cycle.

Unlike the power output during the cure cycle, power output during there-cure cycle did not pass through a local minimum.

DSC analysis on the core samples extracted following the cure cycleconfirmed that all tested regions of the test panels were at least 99%cured.

The difference between the power outputs during the cure and the re-curecycles is shown by line 29 in FIG. 5. This divergence between the poweroutputs during the cure cycle and re-cure cycle can therefore beattributed to heat liberated by the exothermic epoxy resin curingreaction within the test panel. The required power output to follow thepredetermined temperature vs time profile during the re-cure cycle canbe regarded as that expected for heating an article having analogousconductivity and heat capacity, taking into account heat losses to theenvironment for this particular experimental setup.

The initial divergence evident from around t≈22 mins and characterisedby a faint shoulder 30 in line 23, is attributable to the onset of thethermally initiated curing reaction. In further experiments (data notshown) a slower temperature ramp rate was used and the onset of curingmanifested as a “double minimum” was observed in the power output duringcuring.

Whilst absolute power output varied, due to differences in heatingelement power rating, RTD accuracy and increased heat loss near theedges of the test panels (evident in data from nodes 1 and 11 inparticular), the difference between cure and re-cure cycles illustratedin FIG. 5 was observed for all nodes in experiments conducted on each ofthe six test panels.

These data indicate that the exothermic response provides a reproduciblecharacteristics that may be tracked. Two such reproduciblecharacteristics are the approach towards constant power output at t≈120minutes and the local minimum observed in the test experiments at t≈78minutes.

Detecting the constant power output of the heat source and/or theapproach thereto, is complicated by the differences between absolutepower output when this steady state is achieved, due to theaforementioned differences in heat loss from different areas of thetool.

However, the constant power output is characterised by the firstderivative of heat output vs time data approaching zero, regardless ofthese absolute values and can be used to determining the curingcompletion time.

An example of the first derivative of heat output vs time data (anexample of functionalised data) is shown in FIG. 6, by line 31. Forcomparative purposes, the heat output vs time data from which line 31was obtained, is shown as line 33. Qualitatively similar results wereobtained for all 11 nodes, and indicates that this approach to a firstderivative of zero, during a dwell period can be used as a reproduciblecharacteristic for determining the curing completion time.

It has also been observed that the local minimum 35 in the firstderivative, at t≈62 minutes (corresponding to the steepest gradient ofthe heat output vs time 33 between the local maximum 37 and the localminimum 35) is also a suitable reproducible feature of the firstderivative of heat output vs time data.

In addition, it has been observed that the time X between the localminimum 35 and the curing completion time (shown at t=120 minutes inthis example) is also reproducible, such that detection of the localminimum, in this example at around 62 mins, can be used to predict whenthe curing completion time will occur. i.e. In this case, X is around 58minutes.

Two-Sided Test Tool

Experiments were also conducted using a further embodiment of a curingtool 101. Features in common with the tool 1 are provided with likereference numerals, incremented by 100 and 200 for the respectivemoulds. The tool 101 comprises both an upper mould 103 and a lower mould203, each of which have an array of 25 nodes 102, 202; as shownschematically in FIG. 7. As previously, each node 102, 202 has a heatsource 105, 205 and a temperature sensor 107, 207.

Each mould 103, 203 was constructed generally as described above inrelation to the mould 3. However, unlike the mould 3 of the one sidedtool 1, both the heat sources 105, 205 and the temperature sensors 107,207 (as well as their respective microcontrollers 111, 211) of each node102, 202 was attached to the outer face of the tool the temperaturesensor and heater element, to facilitate repair/replacement.

Heat output vs time data for node 17 of the lower mould 203 and data forthe upper mould 103 is shown in FIG. 8. Comparable results were obtainedfrom the remaining nodes of both moulds 103, 203.

FIG. 8(a) shows raw heat output vs time data (presented as % of maximumpower of the heater pad). FIGS. 8(b)-(d) show smoothed first derivativedata, in which the raw data has been averaged for an evaluation periodof (b) 1 minute, (c) 5 minutes and (d) 9 minutes. The resulting smootheddata is presented as data blocks each of which has a value of theaverage extends for an evaluation period in the time dimension. The cureprofile was as described above and the start of the dwell period ismarked by line 40.

By virtue of the reduced heat losses from the use of both upper andlower moulds 103, 203, the heat output vs. time data (FIG. 8(a)) fornode 17 of the lower mould (line 41) and the upper mould (line 43) eachequilibrate towards the end of the dwell period at a much lower % powerthan for the nodes of the one sided tool 1 discussed above. The heatsource of node 17 of the lower mould 103 equilibrates at around 2.5% ofits maximum power, and the heat source of the node 17 of the upper mould203 equilibrates at about 5% of its maximum power.

A further consequence of the use of two moulds is that overall heatoutput form the nodes is lower such that a “double minimum” is observedin the heat output vs time data. A first minimum 45 occurs during thetemperature ramp, and arises from heat liberated following the thermalinitiation of the curing reaction, and the second local minimum 47results from heat liberated by the curing reaction during the earlystages of the dwell period.

The curing completion time for node 17 was determined from the smoothedfirst derivative data shown in FIG. 9(d) as follows. The reproduciblecharacteristic was the approach of the first derivative of heat poweroutput towards zero, and curing completion time was determined as whendata blocks from both the upper and lower nodes fell within a thresholdrange of zero.

In this instance, the threshold range T_(r) was set at ±0.001, which iscorrelated to ≥99% completion of the curing reaction. In the exampleshown, the curing completion time was determined at the data blockcentred at t=100 minutes, marked with reference numeral 48. Formanufacturing applications, this would enable this node to be cooled onor after this time. It will be understood that for some applications, afurther 1-2 evaluation periods may be allowed to elapse beforecooling—for example to allow determination of the curing completion timeat other nodes.

In any case, by identifying the curing completion time in this way, theheating may be discontinued far earlier than the t=175 minutes used inthe standard cure profile (FIG. 4).

The threshold value of the first derivative depends primarily on theacceptable percentage completion of the curing reaction and was selectedto be particularly low in the test experiments, so as to berepresentative of the requirements for aerospace applications.

The length of the evaluation period will be dependent on experimentalconditions, and in particular on the amount of noise in the data.

The evaluation period was determined through analysis of cure cycletemperature and power data from experiments using the same resin systemand cure cycle. The power consumption was analysed over the time seriesand changes in the different cure cycle stages were observed to alignwith changes in the first order derivative.

Experiments were selected with heater pad and temperature sensor pairsthat produced a temperature set point tracking with a standard deviationof less than 1.5° C.

The temperature profiles were used to filter the power consumptionprofiles by normalising the oscillations in power consumption based onthe deviations of temperature from the set point. This was conducted ondifferent time periods until the power consumption standard deviationwas reduced to within 5% of rated power for the profile. The optimaltime period for the particular resin system described above was found tobe 9 mins. Accordingly, the evaluation period can be quantitatively orsemi-quantitatively determined from noise or s/n in the temperaturedata.

The curing completion time for node 17 was also determined from thesmoothed first derivative data shown in FIG. 9(c) as follows. Thereproducible characteristic was the local minimum of the firstderivative of heat power output (corresponding to the local minimum 47of the raw power vs time data shown in FIG. 8(a)). It was found to bepossible to identify the local minimum by a data block from both upperand lower moulds having a first derivative value below a threshold valueT_(v), in this case of <0.01, wherein the preceding and following datablocks have higher first derivative values. These criteria weresatisfied for the data blocks centred at t=47.5 minutes (marked withreference numeral 49).

Thus, by t=58 minutes (reference numeral 51), the local minimum 49 couldbe identified, and from this the curing completion time predicted, asbeing a known time period (+50 minutes) from the identified localminimum in the smoothed first derivative.

It should be noted that the local minimum 45 could also be identified att=46 minutes, by identifying a trend in the first derivative values ofsuccessive data blocks, applying a criterion of two successive fallsfollowed by two successive increases.

FIG. 9(a) illustrates that the evaluation period must be selected to besuitable for the amount of noise present in the raw power vs time data.In this example, an evaluation period of 1 minute was not sufficient forthe smoothed data to be sufficiently stable for reproduciblecharacteristics to be identified.

It is to be understood that the embodiments of the method and apparatusdescribed above are illustrative examples of the invention and thatnumerous modifications may be made by one skilled in the art withoutdeparting from the scope of the appended claims.

1. A method of curing a composite article, comprising; providing a heatsource for heating the composite article; detecting atemperature-related property proximal to the heat source; regulating theheat output of the heat source to the composite article, based at leastin part on the detected temperature-related property, to a predeterminedtemperature vs. time profile; acquiring heat output vs time data;functionalising heat output vs time data; and determining a curingcompletion time based on the functionalised heat output vs time data. 2.The method according to claim 1, wherein the heat output is a value ofan electrical current or power supplied to the heat source.
 3. Themethod according to claim 1, wherein functionalising comprises applyingone or more mathematical functions to the heat output vs time data. 4.The method according to claim 1, comprising identifying a reproduciblecharacteristic of the functionalised heat output vs time data, anddetermining the curing completion time based on the reproduciblecharacteristic.
 5. The method according to claim 1, comprisingidentifying a reproducible characteristic of the functionalised heatoutput vs time data, and predicting the curing completion time based onthe reproducible characteristic.
 6. The method according to claim 1,wherein the curing completion time is determined by detecting areproducible characteristic of the functionalised data corresponding toa local maximum or minimum, steepest gradient, or inflection point inthe power output vs time data.
 7. The method according to claim 1,wherein the functionalised heat output vs time data is a firstderivative of the heat output vs time data.
 8. The method according toclaim 7, wherein the curing completion time is determined by detectingwhen the first derivative remains within a threshold range of zero for apredetermined period.
 9. The method according to claim 8, wherein thethreshold range is within ±0.1, within ±0.01, or within ±0.001 of zero.10. The method according to claim 7, wherein the curing completion timeis determined by detecting a local maximum or minimum in the firstderivative heat output vs time data
 11. The method according to claim 1,comprising smoothing the heat output vs time data, either before or moreafter the functionalisation.
 12. The method according to claim 11,comprising calculating a rolling average of the heat output vs time dataor the functionalised heat output vs time data.
 13. The method accordingto claim 11, comprising acquiring heat output vs time data, orfunctionalised heat output vs time data, for an evaluation period andthen averaging said acquired data.
 14. The method according to claim 13,comprising acquiring the first derivative of the heat output vs timedata for an evaluation period, averaging the first derivative vs timedata over the evaluation period to obtain a smoothed data block, andrepeating until a reproducible characteristic is observed in thesmoothed data.
 15. The method according to claim 14, wherein thereproducible characteristic is a smoothed data block having a firstderivative value within a threshold range of zero.
 16. The methodaccording to claim 14, wherein the reproducible characteristic is asequence of smoothed data blocks having first derivative valuesindicative of a local maximum or minimum.
 17. The method according toclaim 13, wherein the evaluation period is around 5 minutes or around 9minutes.
 18. The method according to claim 1, comprising providing morethan one node, each node comprising a heat source and a temperaturesensor.
 19. The method according to claim 18, wherein detecting thecuring completion time comprises identifying a reproduciblecharacteristic in the data from more than one node.
 20. The methodaccording to claim 19, wherein the nodes are paired, and the curingcompletion time the or each pair of nodes is identified only when a saidreproducible characteristic has been identified in data from both nodes.21. The method according to claim 1, comprising cooling the compositearticle based on the determined curing completion time.
 22. The methodof claim 1, wherein the composite article comprises carbon fibrecomposite.
 23. A curing apparatus for curing a composite article, thecuring apparatus comprising; a curing tool, having at least one mould; aheat source for heating a said composite article; a temperature sensorfor detecting a temperature-related property proximal to the heatsource; and a control arrangement configured to; regulate the heatoutput of the heat source to the composite article, based at least inpart on the detected temperature-related property, to a predeterminedtemperature vs. time profile; acquire heat output vs time data;functionalise the heat output vs time data; and determine a curingcompletion time based on the functionalised heat output vs time data.24. The curing apparatus of claim 23, wherein the control arrangementcomprises one or more controllers, a processing resource and datastorage.
 25. The curing apparatus of claim 23, comprising more than one,or a plurality of nodes, each node comprising a heat source andtemperature sensor, optionally wherein the curing tool comprises saidnodes.
 26. The curing tool of claim 25, wherein each node comprises acontroller, such as a PID controller.
 27. The curing tool of claim 25,wherein the nodes are networked together, so as to share a common datastorage, processing resource and/or power supply.
 28. The curing tool ofclaim 23, wherein the or each heat source and/or temperature sensor isattached to, recessed in or embedded in the mould.
 29. The curing toolof claim 23, comprising two opposed moulds, each of which are providedwith one or more nodes.
 30. The curing tool of claim 23, wherein thecuring tool is an out of autoclave (OOA) curing tool.